1. Field of the Invention
This invention relates to electrical contactors. In particular, there is a carbon fiber contactor that uses an elastomer near the tip to reduce flexing.
2. Description of the Related Art
Various devices and methods of dealing with the design of position sensors are legion. More specifically, contacting position sensors have been formed from a contactor tip and a resistive element. The tip moves over the surface of the element and causes an output resistance reading to change. Polymer resistive position sensors are a variable resistor type of electrical devices with a voltage output that converts the linear or angular displacement to an electrical signal. The contactors are conductive moving elements in the sensors that control the voltage output by sliding over a polymer resistive element. These contactors represent a unique type of sliding electrical contacts. They are miniature in design and size. The contactor is typically an alloy of platinum, silver, palladium and gold. The current density is low at approximately 10 A/sqin. or less. They run at slow sliding speeds around 50 fpm or slower. The contactor should be capable of making good electrical contact under all the application environments, basically, heat and cold, dry and humid, dust, vibration, and etc as other types of contacts do. However, the reliability and noise performance requirements are more crucial than with other types of sliding contacts. The contactor must also have good corrosion resistance.
Currently, the contactors of this type are constructed with metallic materials or alloys, because they are good conductors, easy to process and readily available. Noble alloys such as gold, palladium and platinum alloys are often used because of their non-tarnishing character in all. the common air pollutants. These noble alloy contactors have been used for decades, and have been well accepted in the industry. Because of their industry-wide importance the American Society for Testing and Materials has adopted numerous standards for these alloys, as shown in ASTM Standards Vol. 03.04.
As the contactor tip moves against the element surface wear occurs on both surfaces. The prior art contactor tips are formed from a metal having good wear resistance and spring characteristics, for example beryllium copper. The prior art resistive elements have been formed from carbon and polymer combinations that are screened onto a substrate and dried.
There are several common problems occurring with the prior art. As the metal contactor tip moves against the element over time, it can gouge and pit the element causing the resistance reading to fail or to generate an erroneous resistance reading. Another problem with the prior art devices is that over time the element material can be removed from the element and build up as a loose layer over the surface of the element. When the contactor tip is moved it skates across the surface of the loose material instead of making electrical contact with the element. This causes the resistance reading from the element to appear as an open circuit with infinite resistance and essentially causes the sensor to be inoperative for a period of time until contact is reestablished by the contactor tip to the element.
Poor electrical contacts have often been found associated with the metal contactors, even with noble alloy contactors. As a result, electrical noise is generated from these contactors, which lowers the performance and reliability, and ultimately causes failure of the sensor devices. Excessive wear either from the contactor itself or from the polymer resistive elements has also often been found in this type of sensors, which reduces the durability and even causes premature failure of the sensor devices.
Prior art contacting position sensor designers have attempted to solve these problems by introducing a lubricant into the contactor tip/element interface. A typical lubricant is an emulsification of Teflon particles. The lubricant reduces the frictional forces between the wear surface while still allowing electrical contact to be made between the contactor tip and the resistive element. The liquid lubricant is applied to the contact surfaces to reduce the wear. However, most of the liquid lubricants are poor electrical conductors, thereby causing relatively high electrical resistance across contact surfaces and possible failure of the devices where lubricants have been used improperly. The viscosity of the liquid lubricant changes significantly over the application temperature range, resulting in poor contact at low temperatures and insufficient lubrication at high temperatures. There are several other problems that arise from using a lubricant in the contactor system. First, the amount of lubricant applied is critical to the performance of the system. If too little lubricant is applied, the system will have excessive wear. If too much is applied, erroneous resistance readings due to skating will occur. It is very difficult to repeatably apply a uniform layer of the lubricant. Second, using a lubricant adds additional cost in material and labor to the overall sensor cost. Therefore, there is a need for a less expensive sensor unit that can eliminate the use of lubricants and has improved wear characteristics.
Several designs of Carbon fiber contactors are shown in U.S. patent application Ser. No. 09/137,261, filed Aug. 20, 1998 and entitled, xe2x80x9cCarbon Fiber Contacting Position Sensorxe2x80x9d. The contents of which are specifically incorporated by reference in their entirety. While the invention of the foregoing patent application does an admirable job in improving wear resistance and eliminating the need for a lubricant, it has been found that the fibers under some pressure and motion conditions may flex excessively causing what is known as hysteresis. Hysteresis is defined as the percentage error introduced by the contactor as measured by the electrical signal versus the actual position of the contactor. The present invention is directed toward solving this problem.
It is a feature of the invention to provide an electrical contactor. The contactor includes an elongated beam having a U-shaped end. Several carbon fibers form a bundle attached to the beam. The fibers have ends and tips. The ends are compressively held inside the U-shaped end and the tips emanate from the U-shaped end. An elastomer is disposed over the carbon fibers where they emanate from the U-shaped end. The elastomer reduces flexing of the carbon fibers. More specifically, the elastomer is located between the beam and adjacent to the tips with the tips being exposed.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention.